1. Filed of the Invention
This invention relates to starter motors, and more particularly to a start motor comprising a DC motor and an electromagnetic switch for controlling the latter, to start an engine.
2. Discussion of the Related Art
As shown in FIGS. 1 through 3, a conventional starter motor 1 comprises a DC motor 2 and an electromagnetic switch 3 for controlling the latter 2.
The magnetic poles 21 of the DC motor 2 are permanent magnets, which excite the armature 22. The positive and negative brushes 23 and 24 are held in slide contact with the armature 22.
The electromagnetic switch 3 has main contact means 31 which comprises: a movable iron core 32 with a movable contact 303; a pair of normally open stationary contacts 311 and 312 which are provided on one side of the movable contact 303; and a pair of normally closed stationary contacts 321 and 322 provided on the other side of the movable contact 303.
Upon energization of a current coil 33, the movable iron core 32 is attracted through a plunger 35, and it is held attracted by a voltage coil 34 which is connected in series to the current coil 33. The free end of the voltage coil 34 is grounded, while the free end of the current coil 33 is connected to the stationary contacts 312 and 322 which are grounded through the armature 22. The movable 32 core can be moved with the plunger 35.
The connecting point of the current coil 33 and the voltage coil 34 is connected to the positive terminal of a battery 4 through a key switch 5 on the vehicle.
The plunger 35 is engaged with a pinion which is spline- coupled to the output rotary shaft of the armature 22 with a shift lever (not shown).
The stationary contacts 311 and 312 are fixedly secured to a cap 37 which is coupled to the casing 36 of the electromagnetic switch 3. The stationary contact 321 is press-fitted in a core 38 provided in the casing 36 so that it is electrically connected to the core 38. The stationary contact 322 is formed by blanking a copper plate. The stationary contact 322 is connected through a lead wire 316 to the current coil 33, as shown in FIG. 2. The movable contact 303 is moved along a passage 37a formed in the cap 37 of the electromagnetic switch 3.
When, in the starter motor thus constructed, the key switch 5 is closed, the current coil 33 and the voltage coil 34 are energized, so that the movable iron core 32 is moved in the direction of the arrow A, to connect the movable contact 303 to the stationary contacts 311 and 312. As a result, current is supplied from the battery 4 to the armature 22, so that the pinion is rotated to start the engine.
After the start of the engine, the key switch 5 is opened, the voltage coil 34 is deenergized to release the main contact means 31. As a result, the pinion is disengaged from the ring gear, while the movable contact is connected to the stationary contacts 321 and 322, to short the voltage generated by the inertial rotation of the armature 22 thereby to quickly stop the armature 22.
In the conventional starter motor, the electromagnetic switch is operated not only to start the motor but also to short the voltage generated by the inertial rotation of the armature (dynamic braking); that is, it is operated twice each starting operation, forming a relatively large quantity of contact abrasion powder. Leakage current flows through the contact abrasion powder thus formed, to damage components (particularly the cap) of the electromagnetic switch.
Furthermore, in the electromagnetic switch, as shown in FIG. 1, the creeping distance L between the stationary contact 311 on the battery side (being positive at all times) and the movable contact (being negative at all times except the starting time), and the creeping distance L.sub.2 between the stationary contact 311 and the stationary contact 321 (being negative at all times) or 322 (being negative at all times except the starting time) are short, so that current leakage may occur during the period other than the engine starting time; i.e. when the engine is in operation or in pause.
Moreover, as indicated by the broken line in FIG. 3(a), the movable contact 303 can turn until it abuts, against the passage in the cap, and the creeping distance (L') is decreased as much. This short creeping distance together with the large quantity of contact abrasion powder allows leakage current to flow between the stationary contact 311 (being positive at all times) on the battery side and the movable contact 303 (being negative at all times except the starting time). More specifically, the stationary contacts 311 and 312 are exposed in the inner wall of the cap 37, and the stationary contact 311 is positive at all times. And, when the coils are deenergized, the movable contact 303 is negative. Therefore, if the movable contact 303 abuts against the cap passage 37a during the deenergization of the coils, current leakage may occur to damage the cap of insulating material.